Frank Drake had a problem. It was the fall of 1961, a year after his pioneering SETI experiment: Project Ozma. Using an 85-foot antenna in Green Bank, West Virginia, Drake had unfurled the intriguing possibility that we might find proof of intelligent beings by simply eavesdropping on their broadcasts. He had spent several weeks pointing his antenna at two nearby stars, tuning a simple receiver to 1420 MH, hoping to detect transmissions.

The interest generated by Project Ozma was considerable, and so Drake was asked to organize a conference in Green Bank to weigh the possibility of actually detecting the aliens. He decided to invite about a dozen scientists and technologists, and knew that the discussions would range from the astronomical (how many good planets are out there?) to the sociological (how long might a technical civilization survive?)

But he needed an agenda, and there was no precedent to turn to. That’s when Drake came up with a simple equation, a string of seven factors that, when multiplied together, provide an estimate of the number of galactic, transmitting societies. If this number was very small, then SETI made no sense. If it was large, then there was a chance of hearing something. This equation solved his agenda problem.

No “official” estimates for the terms of Drake’s equation were produced by the conference, although Drake himself has suggested that the number of galactic societies that are on the air might be ten thousand. But despite the fact that his formulation is hard to evaluate (we still don’t know the values of many terms), it has become a widely accepted tool for considering the question of extraterrestrial intelligence.

Indeed, 55 years after Frank Drake first wrote this equation on a chalkboard, it has arguably attained the status of the second most famous formula in science (after Einstein’s E=mc2), and can be found in every astronomy textbook. It is also a guide for the research programs of the SETI Institute, each of which can be tied to a term in the equation.

Drake may have not found the aliens, but he found an intellectual path to their discovery.